1. Field of the Invention
The present invention relates to a dynamic voltage adjustment device and related power transmission system, and more particularly, to a dynamic voltage adjustment device and related power transmission system capable of ensuring a voltage difference across a remote load being stable when transmitting electricity to the remote load.
2. Description of the Prior Art
Generally, a power system transmits electricity to a remote load via a medium such as a transmission line (e.g. a coaxial cable and a conducting line). However, realistic transmission lines have different non-ideal transmission impedances resulting in different transmission voltage drops when transmitting currents to the remote load through the transmission lines. The different transmission voltage drops may cause the remote load damaged or operated unstably.
Pleases refer to FIG. 1, which is a schematic diagram of a conventional power transmission system 10. The power transmission system 10 comprises a power converter 100, a transmission line 102, a load 104 and a feedback circuit 106. The power transmission system 10 is utilized for transmitting an output voltage VOUT generated by the power converter 100 to the load 104 through the transmission line 102. The power converter 100 comprises an error amplifier 108 and a power conversion unit 110. The power converter 100 utilizes the error amplifier 108 for comparing a difference between a reference voltage VREF and a feedback signal FB from the feedback circuit 106, such that the power conversion unit 110 generates the stable output voltage VOUT. The transmission line 102 comprises a forward transmission line LINE1 and a reverse transmission line LINE2, which are respectively utilized for a forward transmission from the power converter 100 to the load 104 and a reverse transmission from the load 104 to the power converter 100. The feedback circuit 106 consists of resistors R1, R2 and divides the voltage of the output voltage VOUT to acquire the feedback signal FB, i.e.
  FB  =            (                        R          ⁢                                          ⁢          2                          R          ⁢                                          ⁢          1                    )        ×          VOUT      .      
When the power converter 100 starts providing the output voltage VOUT to the load 104, a load current I_LOAD is generated on the forward transmission line LINE1 and the reverse transmission line LINE2. Since the forward transmission line LINE1 and the reverse transmission line LINE2 respectively have a forward transmission line resistance R_LINE1 and a reverse transmission line resistance R_LINE2, a forward voltage difference ΔV1=I_LOAD*R_LINE1 and a reverse voltage difference ΔV2=I_LOAD*R_LINE2 are respectively generated when the load current I_LOAD flows on the forward transmission line LINE1 and the reverse transmission line LINE2. In other words, a voltage drop equal to the forward voltage difference ΔV1 is generated when the load current I_LOAD flows from the power converter 100 to load 104, and a voltage drop equal to the reverse voltage difference ΔV2 is generated when the load current I_LOAD feedbacks from the load 104 to the power converter 100. Therefore, a load output voltage LOAD_VOUT acquired by the load 104 equals subtracting the forward voltage difference ΔV1 from the output voltage VOUT of the power converter 100, i.e. LOAD_VOUT=VOUT−ΔV1. Similarly, a load ground voltage LOAD_GND of a ground of the load 104 is the reverse voltage difference ΔV2 higher than a ground voltage GND of a ground of the power converter 100, i.e. LOAD_GND=GND+ΔV2. Therefore, at a moment of a start or an end of supplying power, the load 104 would suffer a voltage difference equal to forward voltage difference ΔV1 plus reverse voltage difference ΔV2. The instantaneous glitch may damage the power transmission system 10.
Since a feedback point samples the output voltage VOUT close to the power converter 100 in the power transmission system 10, the output voltage VOUT is a function of a difference between the reference voltage VREF and the feedback signal FB, i.e. VOUT=f(VREF−FB). Since the reference voltage VREF is a constant value and the feedback signal FB only includes information of the output voltage VOUT, the power transmission system 10 cannot acquire information of voltage difference ΔV1, ΔV2 generated by the load current I_LOAD flowing through the transmission line 102 and cannot accordingly adjust the output voltage VOUT of the power converter 100, such that the voltage difference ΔV1+ΔV2 generated at the load 104 cannot be adjusted.
Please refer to FIG. 2, which is a schematic diagram of related signals when the power transmission system 10 operates. As shown in FIG. 2, at the moments the power transmission system 10 starts and ends providing the output load current I_LOAD to the load 104, the output voltage VOUT of the power converter 100 respectively rises and falls slightly, but the error amplifier 108 immediately senses the variation of the output voltage VOUT and recovers the output voltage VOUT to a former voltage level via a feedback mechanism. As shown in FIG. 2, during the power transmission system 10 outputting the load current I_LOAD, a voltage drop between the load output voltage LOAD_VOUT of the load 104 and the output voltage VOUT equals the forward voltage difference ΔV1, and a voltage drop between the load ground voltage LOAD_GND of the load 104 and the ground voltage GND of the power converter 100 equals the reverse voltage difference ΔV2. Therefore, the voltage difference across the load 104 equals a difference between the load output voltage LOAD_VOUT and the load ground voltage LOAD_GND, i.e. LOAD_VOUT−LOAD_GND. Therefore, as can be seen from FIG. 2, the load 104 suffers a voltage difference ΔV1+ΔV2 at the moments the power transmission system 10 starts and ends outputting the load current I_LOAD, which may cause the load 104 damaged. Thus, resistance of the transmission line 102 causes the power converter 100 incapable of controlling the load 104 to receive a stable voltage via the negative feedback mechanism. As a result, the load 104 would receive a voltage difference related to the resistance of the transmission line 102 and may be damaged by the voltage difference.
Therefore, for the power transmission system, how to avoid the load generating the voltage difference due to the resistance of the transmission line and allow the load to receive a stable voltage becomes a goal in the industry.